writeback: fix false warning in inode_to_wb()

inode_to_wb() is used also for filesystems that don't support cgroup
writeback.  For these filesystems inode->i_wb is stable during the
lifetime of the inode (it points to bdi->wb) and there's no need to hold
locks protecting the inode->i_wb dereference.  Improve the warning in
inode_to_wb() to not trigger for these filesystems.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: aaa2cac ("writeback: add lockdep annotation to inode_to_wb()")
Signed-off-by: Jan Kara <[email protected]>
Signed-off-by: Andreas Gruenbacher <[email protected]>
Reviewed-by: Andreas Gruenbacher <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Andreas Gruenbacher

include/linux/backing-dev.h

@@ -249,6 +249,7 @@ static inline struct bdi_writeback *inode_to_wb(const struct inode *inode)
 {
 #ifdef CONFIG_LOCKDEP
 	WARN_ON_ONCE(debug_locks &&
+		     (inode->i_sb->s_iflags & SB_I_CGROUPWB) &&
 		     (!lockdep_is_held(&inode->i_lock) &&
 		      !lockdep_is_held(&inode->i_mapping->i_pages.xa_lock) &&
 		      !lockdep_is_held(&inode->i_wb->list_lock)));